Consider the following page reference string:
1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6, 3, 2, 1, 2, 3, 6.
How many page faults would occur for the following replacement algorithms, assuming two or three frames? All frames are initially empty. （ 15 pts ）
（ 1 ） LRU replacement （ 5 pts ）
（ 2 ） FIFO replacement （ 5 pts ）
（ 3 ） Optimal replacement （ 5 pts ）
